The present invention is directed to an improved method of destroying pathogenic organisms, excess biosolids, and/or organic contaminants through the utilization of thermophilic temperatures within sludge and similar materials that include microorganisms such as result from waste water treatment processes. In particular, the systems and methods of the present invention utilize controlled recirculation of air within the sludge to maintain the temperature of the sludge in a pre-selected efficient temperature range for destroying the pathogens.
Over many years numerous processes have been developed for the treatment of waste water which utilize microorganisms for the destruction of various contaminants, especially organic contaminants, carried by the waste water. The microorganisms that feed on and, therefore, destroy the contaminants in the waste water form a biomass and after time this biomass becomes too large for the system, such that part must be removed. Consequently, the biomass along with accompanying amounts of silt and the like must be regularly removed from the process. This biomass is generally referred to as sludge and typically includes a wide range of microorganisms. Some of these microorganisms may be harmful to humans and include pathogenic bacteria, enterovirus and certain protozoan organisms. By definition pathogenic organisms are organisms that may be harmful to humans and, therefore, it is important to control the final disposition of the material (sludge) containing these organisms. In general governmental regulations impose various restrictions upon the distribution of sludge having pathogenic organisms. Although regulations of this type vary on a frequent basis, it is generally the rule that the sludge cannot be simply disposed of by distribution on the surface of the ground or in some other manner that could possibly expose humans or water supplies for humans to the pathogens in the sludge.
Sludge also contains other materials including microorganisms which are not pathogenic in nature. Typically the sludge includes a group of microorganisms that thrive in what is generally referred to as the thermophilic temperature range. These thermophilic microorganisms are normally not harmful to humans and there are both aerobic and anaerobic bacteria that thrive within the thermophilic range. This invention is especially interested in the aerobic microorganisms.
Although the temperature ranges for high activity for classification of bacteria varies somewhat depending upon the author describing the range, thermophilic activity usually takes place within the range of from 50.degree. to 70.degree. centigrade. The pathogenic bacteria usually live within what is referred to as a mesophilic range which is around 37.degree. centigrade or the normal body temperature of humans.
An interesting phenomena which has been previously noted by others and used in various sludge processes is that if the temperature of the sludge is raised to the thermophilic range, then over time the mesophilic microorganisms will be destroyed. A further advantage of the thermophilic microorganisms is that their use of oxygen in an aerobic process is exothermic in nature, and consequently, produces heat. Further, although the pathogens can be destroyed by external application of heat to the sludge, this is a very energy intensive process and is normally not justified from a cost point of view. To avoid the expense of heating, various prior art concepts have attempted to use the exothermic nature of the thermophilic bacteria to destroy the pathogenic microorganisms within the sludge.
The major problem with prior art processes which have attempted to utilize the heat produced by the thermophilic microorganisms to destroy the mesophilic microorganisms, is that air must be injected into the sludge in order to provide oxygen to react with the thermophilic bacteria. Depending upon the type of sludge and the depth of the sludge, only approximately 2% of the air (about 10% of the oxygen) may react with the sludge as it passes up through the sludge and is exhausted from the process.
The air utilized for such processes is normally compressed air which has been obtained from the ambient air surrounding the processing plant. This air is normally not 100% humidified and may have a temperature which is relatively low compared to the desired thermophilic temperature of the sludge. Consequently, as the air rises through the sludge it will be humidified and heated thus substantially reducing the temperature of the sludge.
Since the thermophilic microorganism reaction rate is very sensitive to the temperature (directly proportional), and because the destruction of the mesophilic microorganisms is directly proportional to the temperature, it is desirable to maintain the temperature near the upper end of the thermophilic range in order to substantially reduce reaction time. As an example, reaction time to destroy pathogens at 54.degree. centigrade may be approximately 100 hours whereas reaction time at 65.degree. centigrade for an equivalent destruction may be approximately 3 hours. Therefore, the flow of fresh air through the sludge has a substantial detrimental effect upon the rate of reaction and the amount of time required to destroy pathogens within a given amount of sludge, since the air significantly reduces the temperature of the sludge.
Furthermore, thermophilic sludge reactions require at least a minimal amount of residence time for the oxygen in the air to react with the microorganisms within the sludge. Since the rate of rise of air bubbles through the sludge is not substantially controllable by any simple mechanism, most efficient processes of this type utilize fairly tall tanks such that the air rises through a relatively high liquid layer and, therefore, has a longer residency in conjunction with the liquid to allow for exchange of oxygen with the bacteria and for reaction to occur. That is, air residence time in a tank that is 50 feet tall would be approximately ten times as long within the liquid as compared to a tank that had a liquid layer that was only five feet tall. Unfortunately, in many localities, the maximum height of a processing vessel is limited due to ground conditions or zoning restrictions and only relatively short tanks can be utilized. The net result is that the air cannot be utilized efficiently by the bacteria in the liquid, but becomes fully saturated and heated by the liquid thereby substantially lowering the temperature of the liquid to the lower end of the thermophilic range or even outside of the thermophilic range.
Processes of this type have, therefore, had only very limited success and in some locations the goal of pathogen destruction has not been met even after extended periods of reaction time.
Even where reaction time is slow, a processing plant may not be able to wait for a substantial passage of time for destruction of the pathogenic organisms by thermophilic reaction, since the waste water treatment plant is typically producing a fairly large amount of sludge which must be removed from that process on a daily basis and, therefore, treated relatively quickly. Otherwise, there would be a requirement for perhaps ten to thirty tanks in order to receive each day's allotment of sludge and provide sufficient time for the treatment of the sludge in the low thermophilic temperature range before it can be pumped and the tank refilled with new sludge.
A further problem that has existed in the prior art associated with thermophilic destruction of pathogens in this manner has been that such processes often include complicated and energy intensive equipment to operate. Some have incorporated extensive mixing equipment within the sludge reaction vessels and/or other equipment which has substantially added to the operating and capital cost of such processes. Furthermore, the prior art has only one acceptable operating level, whereas it is desirable to have equipment that can process different volumes of sludge at variable batch levels and consequent different batch process retention times. This variability is especially important for proper operation, when flow of material to the process varies. Consequently, it is desirable to provide a process of this type utilizing inexpensive and relatively energy efficient processing equipment where possible, especially where all moving parts are outside the reaction vessel. Further processes that have used simple air diffusion without mixing have normally proved to be very inefficient.
With respect to processes for the destruction of waste water contaminants and other organic contaminants, it has been found that operation of a treatment process in the thermophilic range greatly reduces the amount of excess biomass (sludge) production. This in turn reduces the amount of sludge wasted to disposal and the cost of such disposal. Therefore, while the methods and apparatus described herein are especially suitable for treating excess sludge, such methods and apparatus can be utilized for waste water treatment and similar processes reducing organic material within a fluid.